Such systems typically include a compressor for compressing a refrigerant, a condenser for condensing the refrigerant, an evaporator for evaporating the refrigerant, a discharge fluid line interconnecting the compressor and the condenser, a liquid fluid line interconnecting the evaporator and the condenser, and a suction fluid line interconnecting the evaporator and the compressor. Also, such systems utilize various valve arrangements for controlling refrigerant flow through the system during operation and during shutdown and/or in response to a leak of refrigerant in the system. In addition, an accumulator/dehydrator is frequently disposed in the suction line for accumulating refrigerant, or a receiver/drier disposed in the liquid line for storing refrigerant.
Past experience and recent studies demonstrate that at low load conditions, when the variable displacement compressor is operating at part stroke, refrigerant is trapped in the low side of the system, particularly in the evaporator. Minimizing this resident refrigerant will allow the system to be ready for any unforeseen events. One of the ways of minimizing this resident refrigerant is to minimize the internal volume of the evaporator. This entails proper design of the evaporator for low flow conditions. Another key factor in this design is to have very little volume at the bottom of the evaporator. This is particularly true of the U-channel evaporator that has tanks only on one side. These tanks are located at the top of the evaporator preventing any pooling of liquid refrigerant at the bottom of the evaporator.
Despite the efforts of minimization of charge resident in the evaporator, under certain conditions with a variable compressor, some refrigerant that is not actively participating in the heat transfer process stays resident in the evaporator. The reason for this is that the system has excess refrigerant to handle transient conditions and loss of charge over time. At low loads, the compressor de-strokes to match the needs of the system. Thus, the pumping capacity is reduced and the velocities of the refrigerant fall within the system. If the velocity is not high enough within the evaporator to overcome the gravitational effect, the liquid refrigerant tends to pool at the bottom. This is especially true of evaporators that have tanks at the bottom. This liquid refrigerant pooled in the evaporator has no “pull” from either the compressor or the evaporation process because of system low loads and thus stays resident in the evaporator, pull being the natural migration of refrigerant to the coolest areas of the system. Accordingly, there is a constant need for methods to minimize the pooling of refrigerant in the evaporator.